As illustrated in FIG. 16, an optical communication link 3 includes an optical fiber 2 and optical communication modules 1. The optical fiber 2 is provided for transmitting modulated light which is suitable for optical transmission, in accordance with a data signal to be transmitted, and the optical communication modules 1 are connected to the respective ends of the optical fiber 2 so as to be optically coupled therewith.
There are several types of the optical communication link 3, classified according to the modes of communications. The modes of communications are roughly grouped under the following types: {circle around (1)} the optical fiber 2 is either a single fiber or a plurality of fibers; {circle around (2)} a signal is transmitted either bi-directionally or single-directionally; {circle around (3)} a signal is transmitted either simultaneously (in a full-duplex manner) or in a half-duplex manner, etc. Optical communications are generally carried out by using more than one of the foregoing types in combination (e.g. single fiber full-duplex communications).
A full-duplex communications method using a plurality of optical fibers has conventionally had such a problem that the downsizing of an optical communication module is difficult and the costs of an optical fiber increase as the transmission distance is lengthened.
For these reasons, optical communication modules by which full-duplex optical communications can be carried out using a signal optical fiber (single fiber full-duplex method) have been proposed. In particular, since a plastic optical fiber (hereinafter, this will be referred to as POF) recently has realized the reduction of losses and has been adoptable to broadband communications, the single fiber full-duplex optical communication modules have been applied to home networking and communications between electronic devices. As the POF is around 1 mm in diameter so as to be a large-diameter fiber, the same can be easily connected to an optical communication module and hence the use of the POF makes it possible to manufacture an optical communication link in which an optical fiber is easily detachable/attachable from/to an optical communication module.
In the case of an optical communication module which carries out the full-duplex communications using a single optical fiber, a single optical fiber is used for both receiving and transmitting, so it is critical to restrain (preferably prevent) interference between outgoing light and incoming light, that is, light sent out to the second party involved in communications and light sent in from that party. The interference between outgoing light and incoming light occurs primarily in the following four situations: {circle around (1)} The outgoing beams of light are reflected at the end face of an optical fiber, on the occasion of entering the optical fiber. (Hereinafter, this particular reflection will be referred to as “near end reflection.”); {circle around (2)} The outgoing beams of light are reflected at the end face of an optical fiber, when the beams of light which have been transmitted through the optical fiber exits the optical fiber. (Hereinafter, this particular reflection will be referred to as “far end reflection.”); {circle around (3)} The beams of light are reflected in the optical communication module located at the far end of the line. (Hereinafter, this particular reflection will be referred to as “far end module reflection.”); and {circle around (4)} The beams of light are scattered inside the optical communication module. (Hereinafter, this phenomenon will be referred to as “internal scattering.”). In addition, electric and electromagnetic interferences also cause problems.
Moreover, in an optical communication link using an optical fiber as a transmission medium, it is critical to couple incoming light exiting the optical fiber with a light-receiving device highly efficiently, in order to acquire a high SN (signal-to-noise) ratio.
Enlarging the light-receiving surface of the light-receiving device enables to improve reception efficiency. However, since the stray capacitance of the light-receiving device increases as the light-receiving surface thereof is enlarged, it is necessary to reduce the size of the light-receiving surface in order to restrain adverse effects caused by the stray capacitance, as transmission rate increases. On this account, it is difficult to couple incoming light with a light-receiving device in a highly efficient manner.
To couple an optical fiber with a light-receiving device, there is a conventional method arranged in such a manner that an optical system such as a lens and a mirror is provided between the optical fiber and the light-receiving device, and incoming light emitted from the optical fiber is collected so as to be coupled with the light-receiving device.
In particular, Japanese Laid-Open Patent Application No. 63-90866/1988 (Tokukaisho 63-90866; published on Apr. 21, 1988) and Japanese Laid-Open Patent Application No. 2000-180601 (Tokukai 2000-180601; published on Jun. 30, 2000) disclose methods for coupling an optical fiber with a semiconductor device (such as a light-emitting device and a light-receiving device) using a collection mirror having a curved surface such as spheroid. These methods enables to couple an optical fiber with a semiconductor device highly efficiently.
That is to say, the light-emitting point of the optical fiber and the light-receiving device (light-emitting device) are provided at respective two focal points of the spheroid, so that almost 100% of the beams of light emitted from the optical fiber can be collected by the light-receiving device.
However, it is noted that, in the methods disclosed by Japanese Laid-Open Patent Application No. 63-90866/1988 and Japanese Laid-Open Patent Application No. 2000-180601, the light reflected in the light-receiving device is then reflected in the collection mirror again, and hence it is highly likely that the light returns to the optical fiber as reflected light. This reflected light is transmitted via the optical fiber and causes adverse effects on an optical communication module located at the far end of the line. For instance, the return of the reflected light to the light-emitting device located at the far end of the line makes the oscillation of the light-emitting device unstable. Especially, when carrying out the single fiber full-duplex communications, the interferences because of the far-end module reflection increase as described above, and this causes the decrease of the SN ratio.
Moreover, in the case of the forgoing methods using a collection mirror, since it is not possible to simultaneously provide both of a receiving optical system and a transmitting optical system, there is no space available for providing the transmitting optical system when, for instance, a collection mirror is provided as the receiving optical system, and hence it is impossible to adopt these methods to the single fiber full-duplex communications.